After an oil rig drills a well and installs the well casing, the rig is dismantled and removed from the site. From that point on, a mobile repair unit is typically used to service the well. Servicing includes installing and removing inner tubing strings, sucker rods, and pumps. The variety of work requires a myriad of tools.
One piece of equipment that is found on almost every well service rig is a hoist system for controlling the movement of a cable attached to a traveling block. The hoist winds and unwinds a cable that is attached to a traveling block, which is ultimately used for the prime function of a well service rig: to raise and lower heavy objects, such as rods and tubing, into and out of oil and gas wells. The hoist is usually driven by a variable speed engine coupled in part to the hoist. The prime mover (engine) drives the hoist, usually utilizing a chain driven compound and an air actuated friction clutch, the drum clutch being a critical component of the overall hoist system. The clutch is frequently the most often abused component of the overall drum system. For the most part, the abuse comes from unwanted slippage, which leads to excessive wear on the clutch assembly, leading to a reduced load size the well service rig can lift, and ultimately leading to a total breakdown of the rig.
The coupling effect of a clutch is a function of both the frictional component of the clutch (coefficient of friction and cross sectional area) and the total force between the drum and the compound plates. Naturally, higher hook loads supported by the well service rig require increased coupling between the compound and the drum, thereby requiring a stronger clutch. Since well service rig clutches are commonly air actuated, the amount of air pressure being exerted on the clutch assembly is critical to it operating properly.
The life of a drum clutch on a well service rig varies in accordance to usage and operator technique. On some rigs, clutches can last over 5 years, while on others they are replaced at intervals less than 1 year. This leads to costly rig repairs and even costlier rig down time. Therefore, this industry needs a system to assist the rig operator in operating the drum clutch so as to reduce the wear on the clutch caused by operator technique.
Well service rig operation is simple in explanation, but often is difficult to perfect. When pulling out of the well bore, the traveling block is first attached to a piece of tubing in the hole. The operator then pulls the joint out of the hole by engaging the hoist clutch and applying throttle to full RPM in the engine. When the pulled joint is completely above ground—i.e. out of the well—the operator reduces the throttle, sets the slips that hold the suspended tubing left in the hole, and releases the clutch and lets the engine idle. The tong person then engages the rod tongs and starts unscrewing the above ground joint from the below ground suspended joints. The operator then applies engine RPM so as to supply hydraulic power to the tongs.
When the joint is unscrewed, the tong man reverses the tongs and pulls them away from the unscrewed joint. The rig operator then idles the engine, engages the clutch, and carefully eases the unscrewed joint out away from the suspended tubing. Once released, the rig operator disengages the clutch so that the floorhand can reach out, grab the unscrewed joint, and guide the released tubing to its birthing spot on a racking board. The operator joint is then released from the traveling block, after which the rig operator lowers the block to the floor for attachment to the next joint of tubing still in the hole, after which the process is repeated until all the tubing is pulled from the hole.
During this process, the clutch is engaged en disengaged twice, while the engine goes from idle to full throttle two times as well. Several potential problems arise if this operation is not precisely performed or if short-cuts are taken. For instance, if the engine is not allowed to idle between high RPM uses, the pipe and/or tongs can jerk, which can cause serious equipment damage and prevents a great physical danger to workers in the area. Other accelerated movements or jerking actions of the rig clutch system can also present problems to both employees and equipment. Safety of the crew is compromised when the rig is not operated in a smooth and predictable fashion, as abrupt movements with no warning to the crew can cause accidents. In addition, several high wear points on well service rigs, including clutch, drawworks, chains, sprockets, drilling lines, pad indicators, and jack stands suffer undue wear due to sudden an abrupt changes in rig operation.
In observing the actual rig operation, it is apparent the crew must work in unison with the machine and work in unison with each other. Furthermore, it is very beneficial if the process is predictable to all persons involved, so as to maximize safe operation of the rig (e.g., the workers know where to put their hands, when to step back, when to be extra careful, etc.). Breaking the routine is what causes problems with the machine and safety hazards with the workers. Because current systems are not automated to assist the rig operator in smooth operation of the rig, there is a need to provide a system that can provide this assistance to the operator, and ultimately reduce costly repairs to parts and injuries to workers.